1. Field of the Invention
This invention relates generally to liquid purification or separation and more particularly to liquid-solid separation by siphonal filtration through the use of a porous filter medium.
Liquid-solid separations are important in industry, medicine and the sciences. Processes and apparatus for such separations can be divided into two broad categories: (a) those for separating liquids and dissolved solids and (b) those for separating liquids and undissolved solids. This invention provides a process and apparatus for separating liquids and undissolved solids.
2. Description of the Prior Art
The prior art provides siphonage for transporting liquids. The prior art also shows filters. However, the prior art known to Applicant does not disclose a process or apparatus which prevents "cake" formation by undissolved solids in any filter/transport system from obstructing liquid flow. Cake formation in prior art siphons obstructs liquid flow and requires cleaning.
For purposes of this application, "siphon filter" is used to describe a porous medium which is capable of transporting liquid from one level to a lower level by siphonage while substantially preventing the transportation of undissolved solids above a given size. The transportation can be accomplished in two ways. First, the liquid can be drawn out of a container (reservoir up to a higher elevation and then downward to a level below the surface of the liquid. Second, the liquid can be taken from the reservoir directly to a lower level without first rising to a higher elevation.
Siphons can be classified in two categories: (a) closed siphons and (b) open siphons.
Closed siphons are conduits, such as pipes or hoses, usually configured in an inverted U-shape with an intake end and a discharge end. A hydraulic gradient created by the difference in levels between the intake end and the discharge end causes liquid to flow through the siphon.
Open siphons are characterized by the absence of a pre-defined conduit acting as a siphon. Nevertheless, the intake end of an open siphon must pass through the plane of the surface of the liquid. The liquid does not have to be transported further above the liquid surface level by the siphon, but the siphon must at least intersect the interface between the surface of the liquid and the gas above the liquid. The "conduit" of an open siphon filter is the outermost layer of liquid that wets a porous medium. The porous medium is the scaffolding inside of which the envelope of liquid forms. The medium contains multiple interconnected passages. The liquid flows through the medium by siphonage. Above the liquid, the porous medium contacts a gas phase such as vapor of the same liquid, atmospheric gas, or other gases or vapors. Although normally one would prime the siphon by saturating the porous medium, this is not necessary. Porous media can self-prime. Capillary action raises liquid from the surface of the reservoir into the porous media. Once liquid reaches the top portion, a very slow process by capillary action, gravity will pull the liquid down toward the outlet end of the medium. At this point, capillary action is overcome by siphonage and liquid flow increases.
The prior art shows apparatus which use porous media for moving, transferring, supplying, or dispensing liquids to lower levels by siphonage. Examples of porous media are: bundles of fibers, matted filamentous material, micro-porous membranes, and sorbent material. See U.S. Pat. Nos. 2,770,492; 2,515,569; 2,520,056; and 2,457,851. However, these patents do not deal in any way with filtering or liquid solid separation. They do not address the problem of filtering, they do not solve the problems of filtering, and they do not teach selecting a porous medium that is related to excluding undissolved solids of a given size.
U.S. Pat. No. 4,280,658 to Ehrrich discloses a process and apparatus for liquid supplying and liquid cleaning through capillary action. However, Ehrrich does not teach or deal with the separation action of the present invention. His device transports liquid with siphonage, but it filters with capillary action. Filtration takes place at the second vessel where the micro-porous membrane is placed. Capillary action, not siphonage, filters the liquid as it travels through the membrane from the bottom to the surface where it is evaporated. No hydraulic gradient is involved during filtration in Ehrrich's invention.
U.S. Pat. No. 4,126,556 to Swanson discloses an apparatus for removal of an emersed chemical from a chemical mixture. Here, a porous material capable of absorbing the chemical to be separated is positioned as a siphon, and that specific chemical is removed. Although employing the structure of an open siphon, Swanson uses a system capable of siphonage only for transportation. He does not remove solids from a mixture of liquids; he removes one liquid chemical from a mixture of liquid chemicals by using a chemically specific wick. (See column 7, lines 1-5.) Swanson does not suggest separating all liquids from undissolved solids by using the same medium, nor does he deal with any of the clogging problems found with traditional filters.
U.S. Pat. No. 3,236,768 to Litt discloses a structure which could be taken as an open siphon filter, but it does not suggest or teach the present invention. Litt discloses a process for water purification using a capillary lattice. The lattice in Litt comprises a body or mass which is compacted enough not to have any passages of a size which serve merely as siphon conduits. Water, by capillary diffusion and under gravity, moves inwardly and downwardly through the lattice leaving behind ions collected on the lattice's faces. The lattice is a matted filamentous material compacted so much that it eliminates any passages capable of serving as siphon conduits. Additionally, Litt deals with separation of ionic particles, i.e., dissolved solids. He never suggests to apply his invention for separation of undissolved solids. He expressly states that heavy or gross solids can be removed by conventional mechanical filters (see for example, column 2, lines 41-44 and column 3, lines 33-35).
The art knows closed siphon filters for liquid purification or separation; for example an abstract obtained of Japanese Patent No. 55116419 dated 80-09-08 describes a method for producing a filtering element for filtering liquids. That patent teaches capillary forming raw material charged in a straight pipe, bent into a U-shape and placed to form an inverted U-shaped solid siphon pipe. Also, Japanese Patent Nos. 55011056 dated 80-01-25 and 54151565 dated 79-11-28 describe closed capillary filters comprising a number of tubes dipped at one end into a bath of liquid to be filtered, each of the tubes being packed by a bundle of filaments to draw the liquid from the bath into the tube by capillary action. They do not disclose open siphon filters.
Closed siphon filters present the same drawbacks as conventional filters. In those filters, a cake of undissolved solid particles is formed at the intake end of the closed siphon, this cake plugs the closed siphon at the rigid wall and obstructs liquid flow. Consequently, they need frequent cleaning and removal of the cake. The small pressure difference obtained by the hydraulic gradient of a siphon makes the closed siphon filter unsuitable for many applications since the cake requires higher pressure differentials for liquid flow. The pressure friction losses through the cake at the intake end can stop the siphoning action very quickly. Conventional filters require pumps or vacuums to create pressure differences high enough to force liquid past cake.
The present invention overcomes these drawbacks. As the liquid passes into the porous medium, particles which are large enough to be "undesirables" stop at the intake portion of the porous medium and form a cake. As the cake grows, it becomes heavier and falls away. Low flow rates keep the cake from becoming lodged. The most convenient manner in which to obtain the proper flow rate through the open porous medium is by configuring the medium as a siphon, but the proper flow rates needed for laminar flow may be realized through other manners.
It is an object of the present invention to provide a method and an apparatus for separating a liquid from undissolved solid particles through an open siphon filter.
It is another object of the invention to provide a method and an apparatus for filtering a liquid in an easy and economical manner with a wide range of successful applications.
It is another object of the invention to provide a method and an apparatus for filtering a liquid wherein the flow rate is enhanced by decreasing the pressure of a gas contained within the filter.
It is a further object of the invention to provide a method and an apparatus for filtering a liquid with minimal evaporation.
It is another object of the invention to provide a method and an apparatus for filtering liquid wherein a gaseous atmosphere of a desired composition contacts the open siphon filter.
Other objects and advantages of the invention will be evident to those skilled in the art.